The operating principle of the circulating fluidized bed boiler is to circulate fluidized bed material in such a way that it is separated from flue gases by a cyclone and is returned via a return duct back to the furnace of the boiler. In the return duct, a loop seal is generally used, which can also be called a sand seal. An example of such a boiler is presented in U.S. Pat. No. 6,237,541 to Alliston et al. This boiler also comprises a heat exchanger chamber which is placed in the furnace and through which the hot fluidized bed material from the loop seal passes before it enters the actual fluidized bed inside the furnace. It is also well known to place the heat exchanger chamber in the actual loop seal and to provide the chamber with a fluidizing air supply, as presented in U.S. Pat. No. 4,813,479; EP patent 518 482; U.S. Pat. No. 5,184,671, and U.S. Pat. No. 5,463,968. It is the latter alternative that the present invention relates to, namely a loop seal separated from the furnace and equipped with a heat exchanger chamber.
A problem with boilers of prior art comprising such a loop seal is that the operation of the heat exchanger is not controllable.
For example, in the structure of U.S. Pat. 5,184,671, the fluidized bed material may travel from the inlet conduit through a heat exchanger chamber and an alternative route through a second chamber with no heat exchanger. The operation of the heat exchanger can be adjusted by guiding a part of the material via the second chamber by selecting the fluidizing velocities of both chambers in a suitable ratio.